Liquid crystals are widely used in electronic optical displays. In such display systems, a liquid crystal cell is typically situated between a pair of crossed polarizing plates. For transmissive LCDs light from the backlight source is polarized by a first polarizer and transmitted through a liquid crystal cell, where its polarization state is affected according to the molecular orientation of the liquid crystal that can be controlled by applying a voltage across the cell. Then, light having altered polarization state is transmitted through a second polarizer. By employing this scheme, the transmission of light from backlight source can be controlled and the grayscale can be obtained. The energy required to provide for this control is generally much lower than that required for controlling the emission from luminescent materials used in other types of display such as cathode ray tubes (CRTs). Accordingly, liquid crystal technology is used in a number of electronic imaging devices, including (but not limited to) digital watches, calculators, portable computers, and electronic games, for which small weight, low power consumption, and long working life are important.
The contrast, color reproduction (color rendering), and stable gray scale intensity gradation are important quality characteristics of electronic displays that employ liquid crystal technology. The primary factor determining the contrast of a liquid crystal display (LCD) is the quantity of light transmitted by the display, which is in the dark or “black” pixel state. In addition, this light leakage and, hence, the contrast of an LCD also depend on the direction from which the display screen is viewed. Viewing direction is defined as a set of polar viewing angle θ and azimuthal viewing angle φ. The polar viewing angle θ is measured from display normal direction and the azimuthal viewing angle φ is measured in the plane of the display with respect to an appropriate reference direction. Typically, the optimum contrast is observed only within a narrow viewing angle range centered about the normal to the display and falls off rapidly as the polar viewing angle θ is increased. Various display image properties such as contrast ratio, color reproduction, and image brightness are the functions of the angles θ and φ. In color displays, the leakage problem not only decreases the contrast but also causes color or hue shifts with the resulting degradation of color reproduction.
LCDs are replacing CRTs as monitors for television (TV) sets, computers (such as, for example, notebook computers or desktop computers), central control units, and various devices, for example, gambling machines, electro-optical displays, (such as displays of watches, pocket calculators, electronic pocket games), and portable data banks (such as personal digital assistants or of mobile telephones). It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems related to the effect of viewing angle on the coloration, contrast degradation and inversion are solved, the replacement of traditional CRTs by LCDs will be limited.
Thus, the technological progress poses the task of developing optical elements based on new materials with desired controllable properties. In particular, the necessary optical element in modern visual display systems is an optically anisotropic film that is optimised for the optical characteristics of an individual display module.
Various polymer materials are known in the prior art, which are intended for use in the production of optically anisotropic films. Optical films based on these polymers acquire optical anisotropy through uniaxial extension and coloring with organic dyes or iodine. Poly(vinyl alcohol) (PVA) is among commonly used polymers for this purpose. However, a low thermal stability of PVA based optical films limits their applications. PVA based optical films are described in greater detail in Liquid Crystals—Applications and Uses, B. Bahadur (ed.), World Scientific, Singapore—New York (1990), Vol. 1, p. 101.
Synthetic rigid rod polyelectrolytes are used as model objects structurally close to natural rigid-rod polymers such as deoxyribonucleic acid (DNA), proteins, polysaccharides, which are highly capable of forming well-ordered structures by spontaneous self-assembly, which is fundamental to invoke their biological functions. Since the natural rigid-rod polyelectrolytes are difficult to extract without denaturation, synthetic analogues can be studied to investigate some aspects of self-assembling properties in aqueous solutions. Shear-induced mesophase organization of synthetic polyelectrolytes in aqueous solution was described by Takafumi Funaki, Tatsuo Kaneko et al. in Langmuir, 2004, val. 20, pp. 6518-6520. Water-soluble polymer with a completely rigid-rod elementary structure, poly(2,2′-disulfonylbenzidine terephtalamide) (PBDT), was prepared by an interfacial poly-condensation reaction according to the procedure known in the prior art. GPC measurement showed that the number-average molecular weight, Mn, weight-average molecular weight, Mw, and poly-dispersity (Mw/Mn) were 44 000, 63 500, and 1.4, respectively. Because a concentrated aqueous solution of PBDT (more than 5 wt %) was translucent, authors of this paper made an optical microscopic observation of this solution under the crossed polarizer at room temperature. The strong birefringence of the nematic phase has been shown. These results were observed in the concentration range of 2.8-5.0 wt %. Wide angle X-ray diffraction study indicated that PBDT under the nematic state showed an inter-chain spacing, d, of 0.30-0.34 nm (2θ=28.0±2.0°; θ is diffraction angle), which is constant regardless of the concentration (2.8-5.0 wt %). The d value is shorter than that of the ordinary nematic polymers (0.41-0.45 nm), suggesting that PBDT rods have a strong inter-chain interaction in the nematic state to form the bundle-like structure despite the electrostatic repulsion of sulfonate anions.
Rigid rod, water-soluble polymers are described by N. Sarkar and D. Kershner in Journal of Applied Polymer Science, Vol. 62, pp. 393-408 (1996). The authors of this paper pointed out that these polymers are used for many applications such as enhanced oil recovery. For these applications, it is essential to have an extremely water soluble shear stable polymer that can impart high viscosity at very low concentration for economic reasons. It is known that rigid rod polymers can deliver high viscosity at low molecular weight compared with the traditionally used flexible chain polymers such as a hydrolyzed poly-acrylamides. Polymers with helical or double stranded conformations may be considered as truly rigid rod in solution. New sulfonated water soluble aromatic polyamides, polyureas, and polyimides were prepared via interfacial or solution polymerization of sulfonated aromatic diamines with aromatic dianhydrides, diacid chlorides, or phosgene. Some of these polymers had sufficiently high molecular weight (<200,000), extremely high intrinsic viscosity (˜65 dL/g), and appeared to transform into a helical coil in salt solution. These polymers have been evaluated in applications such as thickening of aqueous solutions, flocculation and dispersion stabilization of particulate materials, and membrane separation utilizing cast films.
The synthesis and solution properties of some rigid-chain, water-soluble polymers (poly[N,N′-(sulfo-phenylene) phthalamidels and poly[N,N′-(sulfo-p-phenylene) pyromellitimide]) are described by E. J. Vandenberg et al in Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 27, pp. 3745-3757 (1989). Poly[N,N′-(sulfo-phenylene)phthalamidles and poly[N,N′-(sulfo-p-phenylene)pyromellitimidel were prepared in water-soluble form and were found to have unique solution properties, similar in some respects to xanthan. The most investigated polymer, poly[N,N′-(sulfo-p-phenyl-ene)terephthalamide] (PPT-S), is produced as the dimethylacetamide (DMAC) salt by the solution polymerization of 2,5-diaminobenzenesulfonic acid with terephthaloyl chloride in DMAC containing LiCl. The isolated polymer requires heating in water to dissolve; the resulting cooled solutions are viscous or gels at concentrations as low as 0.4%. They are highly birefringent, exhibit circular dichroism properties, and are viscosity-sensitive to salt. Solutions of this polymer mixed with those of guar or hydroxyethyl cellulose give significantly enhanced viscosity. The polymer is relatively low molecular weight, ca. 5000 estimated from viscosity data. Some meta and para isomeric analogs of PPT-S were prepared; these polymers have similar properties except they are more soluble in water, and higher concentrations are required to obtain significant viscosity. Poly[N,N′-(sulfo-p-phenylene) pyromellitimide] (PIM-S) was prepared similarly from 2,5-diaminobenzenesulfonic acid and pyromellitic dianhydride. Its aqueous solution properties are similar to those of PPT-S. It appears that these relatively low-molecular-weight rigid-chain polymers associate in water to form a network that results in viscous solutions at low concentrations.
Effect of phenylene content on optical, spectral, and solubility properties is described by H. G. Rogers, R. A. Caudiana in Journal of Polymer Science: Polymer Chemistry Edition, vol. 23, pp. 2669-2678 (1985) and in U.S. Pat. No. 4,521,588. In these publications an optical device includes a transparent molecularly oriented highly birefringent polymer. Said highly birefringent polymer comprises repeating molecular units exhibiting high electron density substantially cylindrically distributed about the long axes of the polymer and the repeating units thereof. The highly birefringent polymer is substantially optically uniaxial exhibiting only two indices of refraction. The molecularly oriented highly birefringent polymer comprises recurring units of the formula:

wherein R and R1 are each hydrogen, alkyl, aryl, alkaryl or aralkyl, and A is a divalent radical selected from the group consisting of
(1) a radical

where U is a substituent other than hydrogen, each W is hydrogen or a substituent other than hydrogen, p is an integer from 1 to 3, each X is hydrogen or a substituent other than hydrogen and r is an integer from 1 to 4, said U, Wp and Xr substitution being sufficient to provide the radical with a non-coplanar molecular configuration; and
(2) a radical

where each of Y and Z is hydrogen or a substituent other than hydrogen and each t is an integer from 1 to 4, with the proviso that when each Z is hydrogen, at least one Y substituent is a substituent other than hydrogen positioned on the corresponding nucleus ortho with respect to the

The moiety of the radical, Z and Yt substitution are sufficient to provide the radical with a non-coplanar molecular configuration.
While the monomer or monomers to be polymerized can be dissolved in a suitable amide or urea solvent and allowed to react with formation of the desired polymeric material, a preferred reaction sequence where a mixture of copolymerizable monomers is utilized involves the preparation of a solution of a first monomer in the amide or urea solvent and the addition thereto of a second or other monomer or a solution thereof in a suitable organic solvent, such as tetrahydrofuran. External cooling of the resulting reaction mixture provides the desired polyamide material in high molecular weight and minimizes the production of undesired side reactions or by-products.
The polyamide materials prepared as described can be recovered by combining the polymerization reaction mixture with a non-solvent for the polymer and separating the polymer, as by filtration. This can be effectively accomplished by blending the polymerization mixture with water and filtering the solid polyamide material. The polyamide can be washed with an organic solvent such as acetone or ether and dried, for example, in a vacuum oven.
The polymeric materials utilized in the devices according to these references can be formed or shaped into various films, sheets, coatings, layers, fibrils, fibers or the like. For example, a solution of a substituted polyamide as described hereinbefore, in a solvent material such as N,N-dimethylacetamide, optionally containing lithium chloride solubilizing agent, can be readily cast onto a suitable support material for the formation of a polymeric film or layer of the polyamide material. The polymeric film can be utilized for the production of a birefringent polymeric film or sheet material which can be utilized in an optical device of the invention. Thus, a polymeric film or sheet material can be subjected to stretching so as to introduce molecular orientation and provide a film material having a highly birefringent character.
Known shaping or forming methods can be utilized for the orientation of polymeric materials according to these references. Preferably, this will be accomplished by unidirectional stretching of a polymeric film, by extrusion of the polymer into a sheet, layer or other stretched form, or by the combined effects of extrusion and stretching. In their oriented state, the polymers utilized herein exhibit unusually high birefringence.
Extensive investigations aimed at developing new methods of fabricating dye-based films through variation of the film deposition conditions have been described in U.S. Pat. Nos. 5,739,296 and 6,174,394 and in published patent application EP 961138. Of particular interest is the development of new compositions of lyotropic liquid crystals utilizing modifying, stabilizing, surfactant and/or other additives in the known compositions, which improve the characteristics of LC films.
There is increasing demand for anisotropic films with improved selectivity in various wavelength ranges. Films with different optical absorption maxima over a wide spectral interval ranging from infrared (IR) to ultraviolet (UV) regions are required for a variety of technological applications. Hence, much recent research attention has been directed to the materials used in the manufacturing of isotropic and/or anisotropic birefringent films, polarizers, retarders or compensators (herein collectively referred to as optical materials or films) for LCD and telecommunications applications, such as (but not limited to) those described in P. Yeh, Optical Waves in Layered Media, New York, John Wiley &Sons (1998) and P. Yeh, and C. Gu, Optics of Liquid Crystal Displays, New York, John Wiley &Sons, (1999).
It has been found that ultra-thin birefringent films can be fabricated using the known methods and technologies to produce optically anisotropic retardation layers composed of organic dye LLC systems. In particular, the manufacture of thin crystalline optically anisotropic layers based on disulfoacids of the cis- and trans-isomeric mixtures of dibenzimidazoles of naphthalenetetracarboxylic acid has been described by P. Lazarev and M. Paukshto, Thin Crystal Film Retarders (in: Proceeding of the 7th International Display Workshops, Materials and Components, Kobe, Japan, Nov. 29-Dec. 1 (2000), pp. 1159-1160).
This technology makes it possible to control the direction of the crystallographic axis of an optical film during application and crystallization of LC molecules on a substrate (e.g., on a glass plate). The obtained films have uniform compositions and high molecular and/or crystal ordering with, which makes them useful optical materials, in particular, for polarizers and birefringent films or retarders (compensators).